Ferroelectric metal oxide ceramic materials such as lead zirconate titanate (PZT) have been investigated for use in ferroelectric semiconductor memory devices. Other ferroelectric materials, for example, strontium bismuth tantalate (SBT) can also be used. FIG. 1 shows a conventional ferroelectric memory cell 105 having a transistor 130 and a ferroelectric capacitor 140. An electrode 142 is coupled to a plateline 170 and another electrode 141 is coupled to the transistor which selectively couples or decouples the capacitor from a bitline 160, depending on the state (active or inactive) of a wordline 150 coupled to the transistor gate.
The ferroelectric memory stores information in the capacitor as remnant polarization. The logic value stored in the memory cell depends on the polarization of the ferroelectric capacitor. To change the polarization of the capacitor, a voltage which is greater than the switching voltage (coercive voltage) needs to be applied across its electrodes. An advantage of the ferroelectric capacitor is that it retains its polarization state after power is removed, resulting in a non-volatile memory cell.
FIG. 2 shows a cross-section of a ferroelectric capacitor-over-plug (COP) structure 207. The structure comprises a capacitor 240 having a ferroelectric layer 246 disposed between bottom and top electrodes 241 and 242. A plug 215 is coupled to the bottom electrode. The plug is coupled to, for example, a diffusion region of a transistor, forming a memory cell. Typically, a barrier layer 248 is provided between the plug and the electrode of the capacitor to inhibit diffusion of oxygen which can oxidize the plug.
During the reactive ion etch (RIE) process which forms the capacitor, a thin conductive layer 291 (e.g., conductive fence) forms around the capacitor. This conductive layer shorts the top and bottom electrodes of the capacitor, rendering the capacitor defective. The conductive fence forms because of the sputtering nature of the metal RIE process. Once the fence forms, it is very difficult to remove during further etching process because of the vertical profile.
From the foregoing discussion, it is desirable to provide a capacitor which avoids the shorting problem caused by a conductive fence formed during processing.